TCOM 507 Class 2

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Launching, Orbital Effects Satellite
Subsystems Joe Montana IT 488 - Fall 2003
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Agenda

• Satellite Subsystems

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SPACECRAFT SUBSYSTEMS

• Attitude and Orbital Control System (AOCS)
• Telemetry Tracking and Command (TTC)
• Power System
• Communications System
• Antennas

More usually TTCM - Telemetry, Tracking,
Command, and Monitoring
We will look at each in turn
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AOCS

• AOCS is needed to get the satellite into the
  correct orbit and keep it there
• Orbit insertion
• Orbit maintenance
• Fine pointing
• Major parts
• Attitude Control System
• Orbit Control System

Look at these next
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ORBIT INSERTION - GEO
TWO BASIC TYPES OF GEO INSERTION

• High-Energy Apogee Kick Motor firing
• A few minutes, symmetrical about apogee
• Low-Energy AOCS burn
• Tens of minutes to gt one hour burns, symmetrical
  about apogee
• Uses Dual-Mode thrusters i.e. thrusters used for
  both orbit raising and attitude control

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ORBIT MAINTENANCE - 1

• MUST CONTROL LOCATION IN GEO POSITION WITHIN
  CONSTELLATION
• SATELLITES NEED IN-PLANE (E-W) OUT-OF-PLANE
  (N-S) MANEUVERS TO MAINTAIN THE CORRECT ORBIT
• LEO SYSTEMS LESS AFFECTED BY SUN AND MOON BUT MAY
  NEED MORE ORBIT-PHASING CONTROL

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ORBIT MAINTENANCE - 2

• GEO STATION-KEEPING BURNS ABOUT EVERY 4 WEEKS FOR
  ? 0.05o
• DO N-S AND E-W ALTERNATELY
• N-S REQUIRES ? 10 ? E-W ENERGY
• RECENT APPROACH USES DIFFERENT THRUSTERS FOR E-W
  AND N-S

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FINE POINTING

• SATELLITE MUST BE STABILIZED TO PREVENT NUTATION
  (WOBBLE)
• THERE ARE TWO PRINCIPAL FORMS OF ATTITUDE
  STABILIZATION
• BODY STABILIZED (SPINNERS, SUCH AS INTELSAT VI)
• THREE-AXIS STABILIZED (SUCH AS THE ACTS, GPS,
  ETC.)

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DEFINITION OF AXES - 1

• ROLL AXIS
• Rotates around the axis tangent to the orbital
  plane (N-S on the earth)
• PITCH AXIS
• Moves around the axis perpendicular to the
  orbital plane (E-W on the earth)
• YAW AXIS
• Moves around the axis of the subsatellite point

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DEFINITION OF AXES - 2
Earth
o
Equator
s
Yaw Axis
Roll Axis
Pitch Axis
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TTCM

• MAJOR FUNCTIONS
• Reporting spacecraft health
• Monitoring command actions
• Determining orbital elements
• Launch sequence deployment
• Control of thrusters
• Control of payload (communications, etc.)

TTCM is often a battle between Operations (who
want every little thing monitored and Engineering
who want to hold data channels to a minimum
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TELEMETRY - 1

• MONITOR ALL IMPORTANT
• TEMPERATURE
• VOLTAGES
• CURRENTS
• SENSORS
• TRANSMIT DATA TO EARTH
• RECORD DATA AT TTCM STATIONS

NOTE Data are usually multiplexed with a
priority rating. There are usually two telemetry
modes.
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TELEMETRY - 2

• TWO TELEMETRY PHASES OR MODES
• Non-earth pointing
• During the launch phase
• During Safe Mode operations when the spacecraft
  loses tracking data
• Earth-pointing
• During parts of the launch phase
• During routine operations

NOTE for critical telemetry channels
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TRACKING

• MEASURE RANGE REPEATEDLY
• CAN MEASURE BEACON DOPPLER OR THE COMMUNICATION
  CHANNEL
• COMPUTE ORBITAL ELEMENTS
• PLAN STATION-KEEPING MANEUVERS
• COMMUNICATE WITH MAIN CONTROL STATION AND USERS

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COMMAND

• DURING LAUNCH SEQUENCE
• SWITCH ON POWER
• DEPLOY ANTENNAS AND SOLAR PANELS
• POINT ANTENNAS TO DESIRED LOCATION
• IN ORBIT
• MAINTAIN SPACECRAFT THERMAL BALANCE
• CONTROL PAYLOAD, THRUSTERS, ETC.

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POWER SYSTEMS - 1

• SOLAR CELLS
• 1.39kW/m2 available from sun
• Cells 10 - 15 efficient (BOLBeginning Of Life)
• Cells 7 - 10 efficient (EOLEnd of Life)
• SOLAR CELL OUTPUT FALLS WHEN TEMPERATURE RISES
• 2mV/degree C Three-Axis hotter (less efficient)
  than a spinner

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POWER SYSTEMS - 2

• BATTERIES NEEDED
• DURING LAUNCH
• DURING ECLIPSE (lt70mins)
• BATTERY LIMITS
• NiCd 50 (DODdepth of discharge)
• NiH2 70 DOD

NOTE ISS uses 110V bus and will need 110 kW 30
minute eclipses per day 55 kW required from
batteries Solution using Fuel Cells
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POWER SYSTEMS - 3

• BATTERIES ARE CONDITIONED BEFORE EACH ECLIPSE
  SEASON
• BATTERIES DISCHARGED TO LIMIT
• BATTERIES THEN RECHARGED
• TYPICAL NiH2 BATTERY CAN WITHSTAND 30,000 CYCLES
  (AMPLE FOR GEO WOULD BE 5 YEARS IN LEO)

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COMMUNICATIONS SUB-SYSTEMS

• Primary function of a communications satellite
  (all other subsystems are to support this one).
• Only source of revenue
• Design to maximize traffic capacity
• Downlink usually most critical (limited output
  power, limited antenna sizes).
• Early satellites were power limited
• Most satellites are now bandwidth limited.

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SPACECRAFT ANTENNAS

• SIMPLE GLOBAL BEAM, 17O WID LOW GAIN, LOW
  CAPACITY
• REGIONAL NARROW BEAM FROM REFLECTOR ANTENNA,
  TYPICALLY 3o ? 3o OR 3o ? 6o
• ADVANCED MULTIPLE NARROW BEAMS STATIONARY,
  SCANNED, OR HOPPED

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ANTENNA TYPES

• HORN
• Efficient, Low Gain, Wide Beam
• REFLECTOR
• High Gain, Narrow Beam, May have to be deployed
  in space
• PHASED ARRAY
• Complex
• Electronically steered